By comparison, a 60-watt compact fluorescent bulb costs $1.61, a desktop PC adds up to $28.21 and a refrigerator runs you $65.72.

$1.36 is just 88p. It's actually quite a lot more expensive than that to run an iPad in the UK, though, since electricity here is almost twice as expensive (13.7p as opposed to 7.4p per kilowatt hour). Even so, the key point stands up: the cost of running an iPad for a year is less than 5 per cent of the cost of running a desktop PC for a year.

They may not feel like it yet, but tablets are the future of computing. Just as laptop PCs have eaten away at the market share of desktops, so the same thing will happen with tablets and laptops.

And this, fundamentally, is the flaw in arguments that there must be limits to growth. It's an idea that has celebrated its 40th birthday this year: that economic growth requires continuous consumption of resources, which will eventually overshoot the carrying capacity of the earth. A recent publication by the New Economics Foundation restates it:

[I]ndefinite global economic growth is unsustainable. Just as the laws of thermodynamics constrain the maximum efficiency of a heat engine, economic growth is constrained by the finite nature of our planet’s natural resources (biocapacity). As economist Herman Daly once commented, he would accept the possibility of infinite growth in the economy on the day that one of his economist colleagues could demonstrate that Earth itself could grow at a commensurate rate.

Yet this little bit of news shows that to be false - or at the very least, not as self-evidently true as it seems. No-one can deny that moving from one desktop PC to 20 iPads feels a bit like some growth has happened; and yet it requires fewer, not more, resources.

Good news, everyone! Things might carry on getting better. That would be nice.